The present invention relates to a surface acoustic wave filter, and more particularly relates to a technique which achieves favorable filter characteristics of a longitudinally coupled resonator-type surface acoustic wave filter by reducing the loss of the filter and by improving reflection characteristics of the filter.
Surface acoustic wave (SAW) filters employ a surface acoustic wave which is generated due to a piezoelectric effect. Thus, the surface acoustic wave filters are highly reliable as well as compact and lightweight. For this reason, surface acoustic wave filters are widely used as transmit-receive filters, antenna branching filters and the like of mobile communication devices in these years. This type of surface acoustic wave filter is generally composed of a plurality of interdigital transducers (hereinafter shortened to “IDTs”), each for exciting a surface acoustic wave, which are formed on a piezoelectric substrate, and which are electrically or acoustically connected to one another.
A ladder-type structure and a longitudinally coupled resonator-type structure are each known as the structure in which IDTs are connected to one another. In the ladder-type structure, a plurality of IDTs are connected in a ladder shape. In the longitudinally coupled resonator-type structure, a plurality of IDTs are arranged in a propagation path in which their surface acoustic waves propagate, and are acoustically coupled to one another. Generally in a broad band, the longitudinally coupled resonator-type structure has better attenuation characteristics than the ladder-type structure, and has characteristics that it is easy to extract not only unbalanced signals but also balanced signals.
Longitudinally coupled resonator-type surface acoustic wave filters are disclosed in the following patent documents: Japanese Patent Application Laid-open Publication No. 2002-9588 (hereinafter referred to as “Patent Document 1”); Japanese Patent Translation Publication No. 2002-528987 (hereinafter referred to as “Patent Document 2”); Japanese Patent Application Laid-open Publication No. 2003-92527 (hereinafter referred to as “Patent Document 3”); and Japanese Patent Application Laid-open Publication No. 2003-243965 (hereinafter referred to as “Patent Document 4”).